The proteins which are secreted from the restricted part of the epididymis are suggested to sustain sperm maturation. In porcine species, as the potential abilities of sperm for movement and fertilization greatly increase in the corpus epididymis, the secretions in both the caput and corpus epididymis seem to be very important for the sperm maturation. In this study, we have directed our attention to the 40 kDa protein which is detected in the fluid of the distal caput epididymis of boar. It was purified from the porcine cauda epididymal fluid and its cDNA was cloned from the cDNA library of the distal caput epididymis. According to the deduced amino acid sequence, the 40 kDa protein has been identified as procathepsin L. Northern blot analysis showed that the procathepsin L mRNA was most abundant in the distal caput epididymis among the tissues as examined. Consistent with the distribution of the procathepsin L mRNA in the epididymis, the activity of procathepsin L was absent in the fluid of the proximal and mid caput epididymis and first appeared in the distal caput epididymal fluid, whose contents gradually decreased with the passage through the epididymis. These results first appeared in the first distal caput epididymis expresses very high levels of procathepsin L and unusually secretes it into the luminal fluid instead of targeting it to lysosomes. It has been also found that the mRNA of PDGF, which is known to enhance cathepsin L expression in the culture cells, is very high in the mid caput epididymis, which just precedes the site of procathepsin L secretion. This result indicates that PDGF directly regulates the locally restricted expression and secretion of procathepsin L in the epididymis, which is one of the possible mechanisms involved in the functional differentiation in the epididymis.